Increased Storage Capacity for a Method for Long-Term Storage of Information and Storage Medium Therefor

ABSTRACT

The present invention relates to an information storage medium and a method for long-term storage of information.

The invention relates to a method for long-term storage of informationand to an information storage medium for long-term storage.

Currently there are a wide variety of information storage optionsavailable to choose from. With the arrival of the digital era the needfor cheap and efficient information storage systems has been acute andnumerous new technologies have emerged. The proliferation of informationstorage mechanisms, however, has come with certain unforeseenconsequences. Today's information storage systems are highly fragile andsusceptible to damage. Storage mediums such as hard drives and opticaldisks have life spans of merely a few years, and only when they areproperly preserved and maintained. Even older technologies, such aspaper and microfilm have lifespans of only centuries under the bestcircumstances. All of these information storage technologies aresensitive to heat, moisture, acid, etc. and can thus be easily degradedresulting in information loss.

As the need for data storage grows exponentially, the methods used forstoring data have become increasingly vulnerable to destruction andsusceptible to the passage of time. However, many types of informationshould be preserved against natural degradation to ensure continuationof information for generations to come. In the event of naturaldisasters, such as, for example, strong electro-magnetic radiationemitted by the sun, tremendous amounts of data could potentially bedamaged or destroyed. Thus, there is a need for information storage thatis resistant to environmental degradation and can thus store informationover long periods of time.

SUMMARY

It is an object of the present invention to provide a method and mediumfor long term information storage.

This object is achieved with the features of the independent claims.Dependent claims refer to preferred embodiments.

According to a first aspect, the invention relates to a method forstorage of information. The method comprises the steps of providing aceramic substrate; and creating a plurality of recesses in a surface ofthe ceramic substrate by using a laser and/or a focused particle beam inorder to encode information on the ceramic substrate. The plurality ofrecesses have different depths and each depth corresponds to apredefined bit of information.

According to a second aspect, the invention relates to a method forstorage of information. The method comprises the steps of providing aceramic substrate; coating the ceramic substrate with a layer of asecond material different from the material of the ceramic substrate;optionally tempering the coated ceramic substrate to form a writableplate; and creating a plurality of recesses in a surface of the secondmaterial by using a laser and/or a focused particle beam in order toencode information in the second material. The plurality of recesseshave different depths and each depth corresponds to a predefined bit ofinformation.

The coated ceramic substrate may be optionally tempered before and/orafter information encoding to improve the durability of the coatedceramic substrate including the encoded information. Such tempering is,in particular, preferable in case of ultra-long storage of information(e.g. more than 1,000 years) and/or in case of storage underparticularly harsh conditions such as high humidity or in acidicenvironments. It is generally preferred to temper the coated substratebefore information encoding as this allows providing the final coatedsubstrate in the form of a writable plate to customers or end users whothen only have to inscribe the information on the plate. However,depending on the material combinations and/or the inscription techniqueused it may also be preferable to first create the plurality of recessesand to only afterwards temper the coated ceramic substrate including theencoded information. Such final tempering will allow to more easilygenerate the recesses utilizing e.g. a less powerful laser sourcebecause the untempered second material will not be as durable as thetempered second material.

In another alternative, which is applicable to all aspect andembodiments discussed further below, tempering may not be performed as aseparate method step before and/or after information encoding. Rather,certain coating techniques such as high-temperature PVD (physical vapordeposition), CVD (chemical vapor deposition), PECVD (plasma-enhancedchemical vapor deposition) or ALD (atomic layer deposition) may beperformed under sufficiently high temperatures to achieve an in situtempering during coating.

According to a third aspect, the invention relates to a method forstorage of information. The method comprises the steps of providing aceramic substrate; coating the ceramic substrate with two or more layersof different second materials being different from the material of theceramic substrate; and creating a plurality of recesses in the layers ofsecond materials by using a laser and/or a focused particle beam inorder to encode information in the layers of second materials. Theplurality of recesses have different depths and extend into differentlayers of the two or more layers and each depth corresponds to apredefined bit of information.

The coated ceramic substrate may be optionally tempered before and/orafter information encoding to improve the durability of the coatedceramic substrate including the encoded information. Such tempering is,in particular, preferable in case of ultra-long storage of information(e.g. more than 1,000 years) and/or in case of storage underparticularly harsh conditions such as high humidity or in acidicenvironments. It is generally preferred to temper the coated substratebefore information encoding as this allows providing the final coatedsubstrate in the form of a writable plate to customers or end users whothen only have to inscribe the information on the plate. However,depending on the material combinations and/or the inscription techniqueused it may also be preferable to first create the plurality of recessesand to only afterwards temper the coated ceramic substrate including theencoded information. Such final tempering will allow to more easilygenerate the recesses utilizing e.g. a less powerful laser sourcebecause the untempered second material will not be as durable as thetempered second material.

In case of the presence of two or more layers it may also be preferableto temper the partially coated substrate, for example, after havingcoated the ceramic substrate with a first layer of a different material,and to subsequently apply one or more further layers of differentmaterials.

In other words, the invention underlying the aspects 1 to 3 is based onthe idea to utilize depth encoding in combination with extremely durableand stable substrates and/or layered structures. In various experiments,it turned out that it is possible to repeatably create recesses ofdifferent predetermined depths in these materials using a laser and/or afocused particle beam. Since it is also possible to measure these depthsin a later decoding process one may easily encode various bits on aparticular spot of the surface of the coated substrate, said spot havingan area corresponding to the cross-sectional area (parallel to thesubstrate surface) of the recess. For example, a first depth d1 canencode the bit 00, a second depth equal to 2×d1 can encode the bit 01, athird depth equal to 3×d1 can encode the bit 10, and a fourth depthequal to 4×d1 can encode the bit 11. Of course, more than four depthsmay be used to encode even more bits on one and the same spot. Forstable encoding and decoding, it is preferable that the smallestdifference between subsequent predetermined depths (equal to d1 in thisexample) is much greater than, preferably by a factor of 5 and morepreferably by a factor of 10, the standard deviation of the depth d1achieved during creation of the recesses.

In cases where very small depths and depth differences are utilized itmay be difficult to rely on the absolute position of, e.g., the bottomof each recess which may also depend on a thickness variation of thesubstrate and/or one of the other layers. It may thus be preferable toencode the bit information in a relative rather than an absolute depth.For example, each recess may comprise a step with two different depths(a reference depth and an encoding depth) or for each recess a twinrecess with a reference depth may be provided. The bit can then beencoded in the difference between, for example, the encoding depth andthe reference depth. This allows to produce the substrate and theoptional additional layers with less accuracy and to reducemanufacturing costs. Of course, this principle may be extended to two ormore encoding depths being measured with respect to one and the samereference depth. For example, a matrix of 3×3 or 5×5 recesses may allrely on one central reference depth.

The substrate may have any shape and size suitable for storinginformation. For example, the substrate may be rectangular, quadratic,round or may have a polygonal or other shape. The size may vary between1 cm² and 1 m², preferably between 10 cm² and 1,000 cm2, more preferablybetween 50 cm² and 250 cm².

The layer of the second material or the two or more layers of differentsecond materials is/are preferably coated directly onto the ceramicsubstrate, i.e. without any intermediate layer being present, so as toachieve a strong bond between the ceramic substrate and the layer of thesecond material during tempering. However, tempering may generate asintered interface between the ceramic substrate and the layer of thesecond material or the bottommost layer of the two or more layers ofdifferent second materials. The sintered interface may comprise at leastone element from both the substrate material and the second material orthe material of the bottommost layer of the two or more layers ofdifferent second materials because one or more elements from one of thetwo adjacent layers may diffuse into the other layer of the two adjacentlayers. The presence of the sintered interface may further strengthenthe bond between the ceramic substrate and the layer of the secondmaterial or the bottommost layer of the two or more layers of differentsecond materials. Further sintered layers may be present between thevarious layers of different second materials with each sintered layerpossibly containing at least one element from both adjacent layers.

The layer of second material or the two or more layers of differentsecond materials is/are preferably continuous and preferably extend(s)over a large portion (e.g., at least 80% or at least 90%) of, morepreferably the entire ceramic substrate. Preferably, the second materialor the two or more layers of different second materials is/are differentfrom the material of the ceramic substrate, i.e. the second material mayhave a different elemental composition than the material of the ceramicsubstrate or the second material and the ceramic substrate differ interms of their microscopic structure, e.g. their state ofcrystallization or the like.

Tempering is a process which can be performed on certain materials, suchas ceramics and metals, to improve their durability by altering thematerial's underlying physical or chemical properties. The temperingprocess may assist in fixing the second material or the material of thebottommost layer of the two or more layers of different second materialspermanently to the ceramic substrate. In some cases, a portion of thesecond material layer or the bottommost layer of the two or more layersof different second materials may form a chemical bond to the underlyingceramic substrate, such as for example an inter-metallic orinter-ceramic bond. Tempering may improve the adhesion between substrateand second material or the material of the bottommost layer of the twoor more layers of different second materials as well as the hardness ofthe layer of second material or the bottommost layer of the two or morelayers of different second materials by at least 5%, preferably by atleast 10%. Moreover, tempering may create a sintered interface asdiscussed above. Similar effects may be achieved between the two or morelayers of different second materials: adhesion may be improved betweenadjacent layers and the hardness of each of these layers may beincreased. Tempering may take place with or without oxygen.

If tempering is performed in an atmosphere containing oxygen, thesurface or a topmost sub-layer of the layer(s) of the second materialexposed to oxygen may, at least partly, be oxidized. Thus, a metal oxidelayer may be formed on top of the layer(s) of the second material. Thismay further increase the hardness and/or the melting point and/or theresistance against corrosive environment.

Providing a writable plate with a ceramic substrate coated with a layerof second material as described herein allows for information storagethereon which is highly resistant to moisture, electric/magnetic fields,acidic, corrosive substance, etc. such that the encoded writable plateprovides a durability which is unavailable from other commonly usedinformation storage mediums.

Preferably, the two or more layers according to the third aspect eachhave a thickness smaller than 1 μm, preferably smaller than 100 nm, morepreferably smaller than 10 nm.

Preferably, the two or more layers comprise a metal layer and a metaloxide layer adjacent to each other, wherein the metal element of themetal layer and the metal element of the metal oxide layer arepreferably identical. It has turned out that it is particularly easy tomeasure depth differences between a metal layer surface and a metaloxide layer surface by means of interference because in case of exposureto broad band white light a selective reflection of the broadelectromagnetic spectrum occurs or in case of exposure to a narrow bandlaser beam the reflection factor is increased (compare FIG. 6 ).Moreover, using an interface between a metal layer and a metal oxidelayer is particularly advantageous because such a system does not tendto oxidize further which enhances stability of the layers. For thisparticular embodiment it is thus preferred that one of the differentdepths is a depth exposing a metal layer surface and another one of thedifferent depths is a depth exposing a metal oxide layer surface inorder to benefit from the optical difference between these surfacesduring decoding.

Moreover, these material combinations allow also for color effects.Since different parts of the visible spectrum are typically reflectedand/or absorbed by a metal and its corresponding oxide, the apparentcolor of the surface of the coated substrate depends on the depth of therespective recesses. Using different metal/metal oxide combinations itis thus possible to encode a number of different colors by means ofdifferent recess depths. Thus, a polychromatic ceramic microfilm may bemanufactured. Decoding is also particularly simple in this case as onemay simply illuminate the plate with white light and measure the colorresponse. Of course, one may also combine the different approachesencoding some information by means of color and additional informationby means of depth within one and the same material layer (correspondingto one and the same color response e.g. black, shades of grey andwhite).

Of course, the optical properties of the different materials of thevarious layers may also be utilized during decoding in case of othermaterial combinations. It may, for example, be possible to utilize nlayers of n different materials and to allocate each of the n differentdepths to a single one of these n layers for encoding log₂(n) bits ofinformation. During decoding an optical material response may then bemeasured in order to determine the depth rather than performing anactual depth measurement.

Preferably, the plurality of recesses have at least two, preferably atleast three, more preferably at least four, even more preferably atleast five, even more preferably at least six, even more preferably atleast seven, even more preferably at least eight, even more preferablyat least sixteen and most preferably at least thirty-two differentdepths wherein each depth corresponds to a predefined bit ofinformation.

Preferably, each recess is formed by one or more pulses of the laserand/or focused particle beam wherein the depth of each recess iscontrolled by one or a combination of the following parameters: energyof the pulses, duration of the pulses, number of pulses of the laserand/or focused particle beam.

Preferably, the minimum depth difference between the plurality ofrecesses is at least 1 nm, more preferably at least 10 nm, morepreferably at least 30 nm, more preferably at least 50 nm, even morepreferably at least 70 nm, and most preferably at least 100 nm.Preferably, the minimum depth difference between the plurality ofrecesses is at most 5 μm, more preferably at most 1 μm, more preferablyat most 500 nm, more preferably at most 300 nm, even more preferably atmost 200 nm, and most preferably at most 100 nm.

Preferably, the cross-sectional area of each recess is smaller than 100μm², preferably smaller than 1 μm², more preferably smaller than 100nm², even more preferably smaller than 10 nm².

According to a fourth aspect, the invention relates to a method forstorage of information. The method comprises the steps of providing aceramic substrate; coating the ceramic substrate with a layer of asecond material different from the material of the ceramic substrate;and creating a plurality of nanostructures in a surface of the secondmaterial by using a laser and/or a focused particle beam in order toencode information in the second material. The plurality ofnanostructures have different optical properties wherein each opticalproperty corresponds to a predefined bit of information.

Again, the coated ceramic substrate may be optionally tempered beforeand/or after information encoding to improve the durability of thecoated ceramic substrate including the encoded information.

In other words, the invention underlying this fourth aspect is based onthe idea to utilize surface modification encoding in combination withextremely durable and stable substrates and/or layered structures. Invarious experiments, it turned out that it is possible to repeatablycreate nanostructures such as nanoripples having different opticalproperties using a laser and/or a focused particle beam. Since it isalso possible to measure these optical properties in a later decodingprocess one may easily encode various bits on a particular spot of thesurface of the coated substrate. For example, a first orientation of thenanoripples can encode the bit 00, a second orientation of thenanoripples can encode the bit 01, a third orientation of thenanoripples can encode the bit 10, and a fourth orientation of thenanoripples can encode the bit 11. Of course, more than fourorientations of the nanoripples may be used to encode even more bits onone and the same spot.

Preferably, the different optical properties of the plurality ofnanostructures comprise one or more of the following: orientation orpolarization of nano-ripples, frequency or wavelength of nano-ripples,amplitude of nano-ripples. Preferably, the plurality of nano-rippleshave at least two, preferably at least three, more preferably at leastfour, even more preferably at least five, even more preferably at leastsix, even more preferably at least seven, even more preferably at leasteight, even more preferably at least 16 and most preferably at least 32different orientations, polarizations, frequencies, wavelengths oramplitudes and wherein each orientation, polarization, frequency,wavelength or amplitude corresponds to a predefined bit of information.

Each of the following preferred features are, unless specifiedotherwise, applicable to each of the four aspects outlined above.

Preferably, the ceramic substrate of the method for information storagecomprises an oxidic ceramic, more preferably the ceramic substratecomprises at least 90%, most preferably at least 95%, by weight of oneor a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃,V₂O₃ or any other oxidic ceramic material. These materials are known tobe particularly durable under various circumstances and/or to resistenvironmental degradation. Thus, these materials are particularlysuitable for long-term storage under different conditions. It isparticularly preferred that the ceramic substrate comprises one or acombination of Al₂O₃, ZrO₂, ThO₂, SiO₂ and/or MgO. According to thepresent invention, the term “ceramic material” preferably encompassesglass ceramics having an amorphous phase and one or more crystallinephases. Moreover, the above-mentioned ceramic materials may also bepresent in the form of a polycrystalline or monocrystalline material.For example, monocrystalline aluminum oxide (i.e. sapphire) isparticularly suitable as a substrate material in terms of durability asit has a very high melting point and a very high Mohs hardness.

Preferably, the ceramic substrate comprises a non-oxidic ceramic, morepreferably the ceramic substrate comprises at least 90%, most preferablyat least 95%, by weight of one or a combination of a metal nitride suchas CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN;metal carbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC; ametal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄and a metal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi,Mg₂Si or any other non-oxidic ceramic material. These materials areknown to be particularly durable under various circumstances and/or toresist environmental degradation. Thus, these materials are particularlysuitable for long-term storage under different conditions. It isparticularly preferred that the ceramic substrate comprises one or acombination of BN, CrSi₂, SiC, and/or SiB₆.

Preferably, the ceramic substrate comprises one or a combination of Ni,Cr, Co, Fe, W, Mo or other metals with a melting point above 1,400° C.Preferably, the ceramic material and the metal form a metal matrixcomposite with the ceramic material being dispersed in the metal ormetal alloy. Preferably, the metal amounts to 5-30% by weight,preferably 10-20% by weight of the ceramic substrate, i.e. the metalmatrix composite. Particularly preferred metal matrix composites are:WC/Co—Ni—Mo, BN/Co—Ni—Mo, TiN/Co—Ni—Mo and/or SiC/Co—Ni—Mo.

Preferably, the second material comprises at least one of a metal suchas Cr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, V; ora ceramic material such as a metal nitride such as CrN, CrAlN, TiN,TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN; a metal carbide such asTiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC; a metal oxide such asAl₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃; a metal boridesuch as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄; a metalsilicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si; or anyother ceramic material; preferably wherein the second material comprisesCrN, Cr₂O₃ and/or CrAlN. These materials provide sufficient hardness andresistance to environmental degradation. Furthermore, said materials canprovide sufficient visual contrast with the underlying ceramicsubstrate. Moreover, experiments have shown these materials to bestrongly bonded to the substrates mentioned above once being coated e.g.with PVD (physical vapor deposition), sputtering, CVD (chemical vapordeposition), PECVD (plasma-enhanced chemical vapor deposition) or ALD(atomic layer deposition). An additional tempering with or withoutpresence of oxygen can further improve the strength of these bonds.Thus, a durable, permanent connection between the coated layer(s) andthe substrate may be achieved. It is particularly preferred that thesecond material comprises one or a combination of Co, Ni, B₄C, HfC,Cr₂O₃, ZrB₂, CrB₂, SiB₆, Si₃N₄, ThN, CrN, Cr₂O₃ and/or CrAlN.

In the context of the present invention, various material properties mayplay an important role. For one, the materials of both the substrate andthe coating layer need to be sufficiently durable, stable and resistant.Moreover, a strong bond or connection between the coating layer and thesubstrate material is required. Taking all these constraints intoaccount, the following material combinations are particularly preferred:Al₂O₃/CrN, SiO₂/Cr, SiO₂/CrN, Al₂O₃/Co, ZrO₂/ZrB₂, Al₂O₃/SiC,SiB₆/Cr₂O₃, SiC/HfC, BN/ZrB₂, BN/ZrB₂, BN/B₄C, BN/ThN and CrSi₂/Si₃N₄.

Generally, any technique suitable to achieve thin coatings may beutilized for coating the ceramic substrate with the layer of the secondmaterial or the two or more layers of different second materials, e.g.physical vapor deposition, sputtering, chemical vapor deposition, or anyother thin film coating method. Preferably physical vapor deposition isused to coat the ceramic substrate with the layer of second material orthe two or more layers of different second materials. This particularlyallows for reliably providing very thin coating layers whichcontinuously cover the substrate without any defects which could bemisinterpreted as encoded information. Since it may be difficult to usePVD for some of the materials mentioned above it is preferred that,during physical vapor deposition, the ceramic substrate is positionedintermediate a source of the second material or the material of the twoor more layers of different second materials and an electricallyconductive plate and/or wire grating. A plate or grating positionedbehind the ceramic substrate helps to direct the vapor of secondmaterial to adhere to the (non-conducting) ceramic substrate.

Preferably the layer of second material or the two or more layers ofdifferent second materials has/have a thickness no greater than 10 μm,more preferably no greater than 5 μm, even more preferably no greaterthan 1 μm, even more preferably no greater than 100 nm, even morepreferably no greater than 10 nm.

By providing (a) thin layer(s) of the second material(s), the laser orparticle beam removal of localized areas of the second material may beperformed more quickly and effectively. Moreover, much smaller localizedareas may be altered more precisely if the layer(s) of secondmaterial(s) is/are thinner. Thus, the information content per area maybe improved.

Preferably tempering the coated ceramic substrate involves heating thecoated ceramic substrate to a temperature within a range of 200° C. to4,000° C., more preferably within a range of 1,000° C. to 2,000° C. Thetempering process may comprise a heating phase with a temperatureincrease of at least 10 K per hour, a plateau phase at a peaktemperature for at least 1 minute and finally a cooling phase with atemperature decrease of at least 10 K per hour. The tempering processmay assist in hardening the ceramic substrate and/or permanently bondingthe second material to the ceramic substrate.

Preferably the localized areas of the coated substrate are heated forthe writing/encoding process to at least a melting temperature and/or adecomposition temperature of the second material such that the localizedareas of second material are heated to a temperature of at least 3,000°C., even more preferably at least 3,200° C., most preferably at least3,500° C., most preferably at least 4,000° C. For example, CrNdecomposes at a temperature of about 1,500° C. into Cr (solid) and N(gaseous), whereas the melting temperature of Cr is only reached atabout 1,900° C. Yet, the Cr (silver colored) is noticeably differentfrom the CrN (grayish). Alternatively, treating the surface of thecoated substrate with, e.g., a femtosecond-laser may lead to coldso-called Coulomb explosions leading to material ablation.

Preferably the laser is configured to produce laser light having awavelength within a range of 10 nm to 30 μm, preferably within a rangeof 100 nm to 2,000 nm, more preferably within a range of 200 nm to 1,500nm.

Preferably the laser light emitted by the laser has a minimum focaldiameter no greater than 50 μm, more preferably no greater than 15 μm,more preferably no greater than 10 μm, more preferably no greater than 5μm, more preferably no greater than 1 μm, more preferably no greaterthan 500 nm, more preferably no greater than 100 nm, more preferably nogreater than 50 nm, more preferably no greater than 10 nm. A small focaldiameter allows for information to be encoded on the writable plate witha higher density.

Preferably, an ultra-short pulse laser (picosecond, femtosecond orattosecond pulse) is used for encoding information. This allows forachieving minimal focal diameters no greater than 10 μm and structuresno greater than 5 μm width, more preferably no greater than 1 μm, morepreferably no greater than 500 nm, more preferably no greater than 100nm, more preferably no greater than 50 nm, more preferably no greaterthan 10 nm.

The laser beam is preferably directed at predetermined spots on thesurface of the coated substrate for encoding bits at these predeterminedspots by means of a suitable scanning technique, e.g. galvanometricscanners, polygon scanners, digital micro mirror devices, spatial lightmodulators etc. Moreover, suitable optics may be involved. For example,the laser beam may be directed through a microscope objective forprecise positioning. Oil, water and other fluids with a high refractiveindex may be used for the immersion of the optics in this context.

Preferably a particle beam emitted by the focused particle beamequipment has a minimum focal diameter no greater than 5 μm, morepreferably no greater than 1 μm, more preferably no greater than 100 nm,more preferably no greater than 10 nm. An extremely small focal diameterallows for information to be encoded on the writable plate with anultra-higher density.

Preferably the method further comprises the step of reading informationencoded on the writable plate, more preferably using a digital scanner,digital microscope, laser scanning microscope, optical coherencetomography or scanning electron microscope.

Preferably areas of the coated substrate comprise at least 1 Megabyte ofinformation per cm², more preferably at least 10 Megabytes ofinformation per cm², even more preferably at least 100 Megabytes ofinformation per cm², even more preferably at least 1 Gigabyte ofinformation per cm², even more preferably at least 10 Gigabytes ofinformation per cm². A greater information storage density allows forthe storage of large quantities of information.

According to a fifth aspect, the invention relates to an informationstorage medium. The information storage medium comprises a ceramicsubstrate, wherein the surface of the ceramic substrate comprises aplurality of recesses encoding information on the information storagemedium, wherein the plurality of recesses have different depths andwherein each depth corresponds to a predefined bit of information.

According to a sixth aspect, the invention relates to an informationstorage medium. The information storage medium comprises a ceramicsubstrate coated with a layer of a second material and a sinteredinterface between the ceramic substrate and the layer of the secondmaterial, wherein the second material is different from the material ofthe ceramic substrate, wherein the sintered interface comprises at leastone element from both the substrate material and the second material,wherein the layer of the second material comprises a plurality ofrecesses encoding information on the information storage medium, whereinthe plurality of recesses have different depths and wherein each depthcorresponds to a predefined bit of information.

According to a seventh aspect, the invention relates to an informationstorage medium. The information storage medium comprises a ceramicsubstrate coated with two or more layers of different second materialsand a sintered interface at least between the ceramic substrate and thebottommost layer of the two or more layers, wherein the second materialsare different from the material of the ceramic substrate, wherein thesintered interface comprises at least one element from both thesubstrate material and the material of the bottommost layer, wherein theinformation storage medium comprises a plurality of recesses encodinginformation on the information storage medium, wherein the plurality ofrecesses have different depths and wherein each depth corresponds to apredefined bit of information.

Preferably the two or more layers each have a thickness smaller than 1μm, preferably smaller than 100 nm, more preferably smaller than 10 nm.

Preferably the two or more layers comprise a metal layer and a metaloxide layer, wherein the metal element of the metal layer and the metalelement of the metal oxide layer are preferably identical.

Preferably the plurality of recesses have at least two, preferably atleast three, more preferably at least four, more preferably at leastfive, even more preferably at least six, more preferably at least seven,even more preferably at least eight, even more preferably at least 16,and most preferably at least 32 different depths and wherein each depthcorresponds to a predefined bit of information.

Preferably the minimum depth difference between the plurality ofrecesses is at least 1 nm, more preferably at least 10 nm, morepreferably at least 30 nm, more preferably at least 50 nm, even morepreferably at least 70 nm, and most preferably at least 100 nm.Preferably the minimum depth difference between the plurality ofrecesses is at most 5 μm, more preferably at most 1 μm, more preferablyat most 500 nm, more preferably at most 300 nm, even more preferably atmost 200 nm, and most preferably at most 100 nm.

According to an eighth aspect, the invention relates to an informationstorage medium. The information storage medium comprises a ceramicsubstrate coated with a layer of a second material and a sinteredinterface between the ceramic substrate and the layer of the secondmaterial, wherein the second material is different from the material ofthe ceramic substrate, wherein the sintered interface comprises at leastone element from both the substrate material and the second material,wherein the surface of the layer of the second material comprises aplurality of nanostructures, wherein the plurality of nanostructureshave different optical properties and wherein each optical propertycorresponds to a predefined bit of information.

Preferably the different optical properties of the plurality ofnanostructures comprise one or more of the following: orientation orpolarization of nano-ripples, frequency or wavelength of nano-ripples,amplitude of nano-ripples. Preferably the plurality of nano-ripples haveat least two, preferably at least three, more preferably at least four,more preferably at least five, more preferably at least six, morepreferably at least seven, even more preferably at least eight, morepreferably at least sixteen and most preferably at least 32 differentorientations, polarizations, frequencies, wavelengths or amplitudes andwherein each orientation, polarization, frequency, wavelength oramplitude corresponds to a predefined bit of information.

Each of the following preferred features are, unless specifiedotherwise, applicable to each of the fifth to eighth aspects outlinedabove.

Preferably the ceramic substrate of the information storage mediumcomprises an oxidic ceramic, more preferably wherein the ceramicsubstrate comprises at least 90%, even more preferably at least 95%, byweight of one or a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO,Cr₂O₃, Zr₂O₃, V₂O₃ or any other oxidic ceramic material.

Preferably, the ceramic substrate of the information storage mediumcomprises a non-oxidic ceramic, more preferably wherein the ceramicsubstrate comprises at least 90%, even more preferably at least 95%, byweight of one or a combination of a metal nitride such as CrN, CrAlN,TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN; metal carbide suchas TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC; a metal boride such asTiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄ and a metal silicidesuch as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si, or any othernon-oxidic ceramic material.

It is particularly preferred that the ceramic substrate comprises one ora combination of BN, CrSi₂, SiC, and/or SiB₆.

Preferably, the ceramic substrate comprises one or a combination of Ni,Cr, Co, Fe, W, Mo or other metals with a melting point above 1,400° C.Preferably, the ceramic material and the metal form a metal matrixcomposite with the ceramic material being dispersed in the metal ormetal alloy. Preferably, the metal amounts to 5-30% by weight,preferably 10-20% by weight of the ceramic substrate, i.e. the metalmatrix composite. Particularly preferred metal matrix composites are:WC/Co—Ni—Mo, BN/Co—Ni—Mo, TiN/Co—Ni—Mo and/or SiC/Co—Ni—Mo.

Preferably the second material of the information storage mediumcomprises at least one of a metal such as Cr, Co, Ni, Fe, Al, Ti, Si, W,Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, V, a metal nitride such as CrN, CrAlN,TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄, ThN, HfN, BN; a metal carbidesuch as TiC, CrC, Al₄C₃, VC, ZrC, HfC, ThC, B₄C, SiC; a metal oxide suchas Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, V₂O₃; a metalboride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆, ThB₂, HfB₂, WB₂, WB₄; ametal silicide such as TiSi₂, ZrSi₂, MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si orany other ceramic material; preferably wherein the second materialcomprises CrN, Cr₂O₃ and/or CrAlN.

Preferably the layer of second material has a thickness no greater than10 μm, more preferably no greater than 5 μm, even more preferably nogreater than 1 μm, even more preferably no greater than 100 nm, evenmore preferably no greater than 10 nm.

Preferably areas of the coated substrate comprise at least 1 kilobyte ofinformation per cm², more preferably at least 10 kilobytes ofinformation per cm², even more preferably at least 100 kilobytes ofinformation per cm², even more preferably at least 1 Megabytes ofinformation per cm², even more preferably at least 10 Megabytes ofinformation per cm², even more preferably at least 100 Megabytes ofinformation per cm², even more preferably at least 1 Gigabytes ofinformation per cm², even more preferably at least 10 Gigabytes ofinformation per cm². Providing a high information density on the coatedsubstrate allows more information to be stored per plate and can reducethe costs of production.

Preferably the ceramic substrate has the shape of a tablet or a computerreadable disk. A tablet or computer readable disk shape may allowcomputers or digital scanners to easily read the encoded information andto be compatible to existing scanning systems.

The invention further relates to a use of the information storage mediumfor long-term information storage.

Preferably, in use the information storage medium is stored for a periodof at least 10 years, more preferably at least 100 years, morepreferably at least 1,000 years, more preferably at least 10,000 years,even more preferably at least 100,000 years.

The invention further relates to a method for decoding informationencoded on the information storage medium described above. The methodcomprises the steps of providing the information storage mediumdescribed above; measuring the depth of at least a subset of theplurality of recesses or the optical property of at least a subset ofthe plurality of nanostructures; and decoding the bits of informationcorresponding to the measured depths or the measured optical properties.

Preferably, measuring the depth or the optical property is performedusing a laser beam and/or a focused particle beam such as an electronbeam.

Preferably, measuring the depth is based on one or a combination of:interference, reflection, absorption, ellipsometry, frequency combtechnique, fluorescence microscopy such as STED or STORM, opticalcoherence tomography, scanning electron microscopy, digital (immersion)microscopy (using reflected or transmitted light).

Preferably, measuring the optical property is based on one or acombination of: absorption, transmission, reflection, polarization,interference of non-coherent light and/or laser light.

While the above-described methods mostly rely on direct ablation ofmaterial using a laser or particle beam, it should be noted thatalternative methods for creating recesses of different depths in acoating are known and may be utilized instead of the direct ablationtechniques discussed above. For example, the coated substrate may becoated with an additional layer of photoresist which may be exposed tolight or other radiation in order to generate a certain pattern. Afterdeveloping the exposed photoresist the coated substrate together withthe photoresist may be etched in order to ablate material of, e.g., thelayer of the second material from the substrate wherever no developedphotoresist is present. Thus, a pattern of recesses will be created. Inorder to create recesses having different depths, said process has to berepeated several times with the number of etchings taking place at aspecific location corresponding to the depth of the recess at saidlocation. Suitable techniques for such etching processes are known inthe art and described, e.g., in Handbook of Semiconductor ManufacturingTechnology, Second Edition, edited by Robert Doering and Yoshio Nishi,CRC Press. For example, chrome may be wet etched with cerric ammoniumnitrate and certain acids that include perchloric, acetic, nitric andhydrochloric acids.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention will be explained in more detail inthe following text with reference to preferred exemplary embodimentswhich are illustrated in the attached drawings, in which:

FIG. 1 schematically depicts a cross section through an informationstorage medium according to a preferred embodiment of the presentinvention;

FIG. 2 schematically depicts an example of the process of physical vapordeposition coating of the ceramic substrate;

FIG. 3 schematically shows a perspective view of an example of encodinga writable plate with information using a laser;

FIG. 4 schematically depicts a cross section through an informationstorage medium according to a preferred embodiment of the presentinvention;

FIG. 5 schematically depicts the principle of interference in case of ametal/metal oxide layer system;

FIG. 6 depicts a graph of reflectance versus wavelength in case of ametal/metal oxide layer system;

FIG. 7 schematically depicts a cross section through an informationstorage medium according to a preferred embodiment of the presentinvention;

FIGS. 8 a and 8 b depict micrographs at two different magnificationsshowing an exemplary encoding;

FIG. 8 c shows a 3D visualization of a section of the micrograph of FIG.8 b;

FIG. 8 d shows the cross sectional height profile through the micrographof FIG. 8 a;

FIGS. 9 a and 9 b depict micrographs at two different magnificationsshowing an exemplary encoding; and

FIG. 10 depicts an SEM image taken from an exemplary encoding.

In principle, identical parts are provided with the same reference signsin the figures.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a cross section through an informationstorage medium suitable for long-term storage of information accordingto a preferred embodiment of the present invention. The informationstorage medium comprises a ceramic substrate 150 coated with a layer ofa second material 170, the second material 170 being different from thematerial of the ceramic substrate 150. As mentioned above, a sinteredinterface (not shown) may be present between the ceramic substrate 150and the layer of the second material 170 due to the optional temperingprocess. The layer of the second material 170 comprises a plurality ofrecesses 10 (four of which are shown exemplary) having different depths,wherein each depth corresponds to a predefined bit of information. Inthe embodiment shown in FIG. 1 , four bits of information can beencoded. For example, the smallest depth of a recess 10 (or,alternatively, a surface without any recess at all) may correspond tothe code “0000”. The largest depth of a recess 10 extending, forexample, all the way through the second layer 170 to the substrate 150may correspond to the code “1111”. Analogously, each of the intermediatedepths corresponds to a specific predefined bit of information as well.While the depth difference between subsequent codes is shown in FIG. 1to be constant, this does not necessarily to be the case.

Of course, the 4-bit code shown in FIG. 1 is only one specific example.Depending on the thickness of the second layer 170 and the depthdifferences of the various recesses 10 which can be both reliablymanufactured for encoding and reliably measured for decoding more orless bits may be encoded.

In order to produce such an information storage medium, a method forstorage of information is described herein. Initially, a ceramicsubstrate 150 is provided. As schematically shown in FIG. 2 , theceramic substrate 150 is then coated with a layer of a second material170. The layer of second material 170 is preferably no greater than 50μm thick. The writable plate 110 comprising the ceramic substrate 150and the layer of second material 170 may either be stored until readyfor use or may subsequently be encoded with information 120 using, e.g.,a laser or focused particle beam 190. The laser or focused particle beam190 is directed toward the layer of second material 170 and then, e.g.,heats localized areas of the second material 170 which fall within thefocus of the laser or focused particle beam such that recesses are beingformed at these localized areas. This method will now be described inmore detail.

The ceramic substrate 150 which is initially provided may comprise themajority of the material by weight of the writable plate 110. A numberof different materials may be used for the ceramic substrate 150. Incertain configurations the ceramic substrate 150 comprises an oxidicceramic comprising at least one of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO,Cr₂O₃, Zr₂O₃, V₂O₃ or any other oxidic ceramic material. Alternatively,the ceramic substrate may comprise a non-oxidic ceramic comprising atleast one of a metal nitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN,AlN, VN, Si₃N₄, ThN, HfN, BN; metal carbide such as TiC, CrC, Al₄C₃, VC,ZrC, HfC, ThC, B₄C, SiC; a metal boride such as TiB₂, ZrB₂, CrB₂, VB₂,SiB₆, ThB₂, HfB₂, WB₂, WB₄ and a metal silicide such as TiSi₂, ZrSi₂,MoSi₂, MoSi, WSi₂, PtSi, Mg₂Si, or any other non-oxidic ceramicmaterial. The amount of the oxidic or non-oxidic ceramic present mayvary. Preferably the amount of oxidic or non-oxidic ceramic makes up atleast 90% by weight of the ceramic substrate 150. More preferably theamount of the oxidic or non-oxidic ceramic substrate makes up at least95% by weight of the ceramic substrate 150. One preferred configurationis a ceramic substrate 150 comprising at least 90% Al₂O₃ or SiO₂measured by weight.

The second material 170 is formed as a layer on the ceramic substrate150. The layer of second material 170 is a thin layer in comparison withthe thickness of the ceramic substrate 150 (FIG. 1 not to scale), thesecond layer 170 being preferably at most 50 μm thick. The secondmaterial 170 may principally comprise at least one of a metal such asCr, Co, Ni, Fe, Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, V, ametal nitride such as CrN, CrAlN, TiN, TiCN, TiAlN, ZrN, AlN, VN, Si₃N₄,ThN, HfN, BN; a metal carbide such as TiC, CrC, Al₄C₃, VC, ZrC, HfC,ThC, B₄C, SiC; a metal oxide such as Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO,Cr₂O₃, Zr₂O₃, V₂O₃; a metal boride such as TiB₂, ZrB₂, CrB₂, VB₂, SiB₆,ThB₂, HfB₂, WB₂, WB₄; a metal silicide such as TiSi₂, ZrSi₂, MoSi₂,MoSi, WSi₂, PtSi, Mg₂Si or any other ceramic material; preferablywherein the second material comprises CrN, Cr₂O₃ and/or CrAlN.

One preferred configuration is a layer of second material 170 comprisingprincipally CrN, Cr₂O₃ and/or CrAlN.

FIG. 2 illustrates an exemplary method for coating the second material170 onto the ceramic substrate 150 using physical vapor deposition(PVD). In the PVD process the ceramic substrate 150 is placed into aphysical vapor deposition chamber together with a source 160 of secondmaterial 162. A vacuum is drawn on the physical vapor deposition chamberand the source 160 of second material is heated until a significantportion of the second material 162 contained therein is evaporated orsublimated. The airborne particles 164 of second material then dispersethroughout the physical vapor deposition chamber until they contact asurface 152 of the ceramic substrate 150 and adhere thereto.

Although physical vapor deposition is a method commonly used for coatingmetal substrates, coating ceramic substrates can prove challenging forparticles to adhere to. Thus, in order to improve adherence of secondmaterial particles 164 to the ceramic substrate surface 152, aconductive wire mesh or conductive metal plate 180 may be placed on thefar side of the ceramic substrate 150, such that the ceramic substrate150 is positioned in between the wire mesh 180 and the source 160 ofsecond material 162. Such a conductive mesh/plate 180 when conductingcurrent may attract ionized particles of second material 164 which thenencounter the surface 152 of the ceramic substrate 150 and are heldthere against such that they then adhere to the surface 152 of theceramic substrate. This coating process may also be repeated in order tocoat multiple different surfaces of the ceramic substrate as discussedfurther below.

Depositing a layer of second material 170 on the ceramic substrate 150may be performed using other coating methods, such as sputtering orsublimation sandwich coating. Essentially, any method capable ofproducing a layer of second material 170 may be used. The secondmaterial 170 may not necessarily cover the entire ceramic substrate 150.Instead only portions of the ceramic substrate 150 or a singular side152 of the ceramic substrate 150 may be coated with the second material170.

Once the ceramic substrate 150 is coated with a second material 170, thecoated ceramic substrate then preferably undergoes an optional temperingprocess. Tempering is generally understood to be a process whichimproves the strength and/or other qualities of a material. In the caseof ceramics, tempering can involve heating a ceramic item such that thechemical components thereof undergo chemical and/or physical changessuch that the item becomes fixed or hardened. Tempering of the coatedceramic substrate may involve heating the coated ceramic substrate 150to a temperature within a range of 200° C. to 4,000° C., preferablywithin a range of 1,000° C. to 2,000° C. The tempering process maycomprise a heating phase with a temperature increase of at least 10 Kper hour, a plateau phase at a peak temperature for at least 1 minuteand finally a cooling phase with a temperature decrease of at least 10 Kper hour. The tempering process may assist in fixing the second material170 permanently to the ceramic substrate 150. In some cases, a portionof the second material layer 170 may form a chemical bond to theunderlying ceramic substrate 150. After tempering the ceramic substrate150 with the second material 170, the writable plate 110 is formed. Theproperties of the writable plate 110 are determined by the exactmaterials used within the writable plate 110. The writable plate 110 maynow be stored or directly encoded with information 120. As mentionedabove, the coated substrate may, in addition or alternatively, betempered before and/or after information encoding.

FIG. 3 depicts the encoding of information onto the writable plate 110.During encoding, a laser or focused particle beam 190 directs collimatedlaser light or focused particle beam onto a layer of second material 170of the writable plate 110. The laser or focused particle beam alters theportion of second material 170 within the localized area 175 such thatit is (e.g. optically) distinguishable from the surrounding secondmaterial 170. While FIG. 3 schematically shows the laser or focusedparticle beam imprint of a text, it is to be noted that the encoding ofmultiple bits according to the present invention is most suitable fordigital encoding of information. Alternatively, however, the differentdepths may also be used to achieve a color effect which may be used forproviding a colored text or a colored image on the writable plate 110.

Preferably the laser or focused particle beam heats the localized area175 of the second material 170 to at least the melting temperatureand/or decomposition temperature of the second material 170. The meltingpoint of the second material 170 is dependent on the chemicalcomposition thereof. Preferably, heating the localized areas 175 pastthe melting point may involve heating the localized areas to atemperature of at least 3,000° C., more preferably at least 3,200° C.,and even more preferably at least 3,500° C., most preferably at least4,000° C. Imparting these localized areas with such high temperaturesmay cause a rapid expansion of the second material 170 within thelocalized areas 175. This rapid expansion can cause the second material170 within the localized areas 175 to be ablated and/or vaporized.

Suitable laser wavelengths for the laser encoding methods may include awavelength within a range of 10 nm to 30 μm, preferably within a rangeof 100 nm to 2,000 nm, more preferably within a range of 150 nm to 1,500nm. Of further importance is the minimum focal diameter of the laserlight or focused particle beam which dictates the minimum size of eachrecess. Preferably the laser or focused particle source 190 is capableof focusing the laser light or focused particle beam to have a minimumfocal diameter no greater than 50 μm, preferably no greater than 15 μm,preferably no greater than 5 μm, preferably no greater than 1 μm,preferably no greater than 100 nm, more preferably no greater than 10nm.

The form of the writable plate 110 can be determined by the needs of theuser and the types of information 120 to be encoded. In some instances,the writable plate 110 can be formed in a tablet shape for storage,preferably no larger than 200 mm by 200 mm, more preferably no largerthan 100 mm by 100 mm, more preferably no larger than 10 mm by 10 mm. Inother instances a computer readable disk-shape may be preferable with adiameter no larger than 30 cm, more preferably no larger than 12 cm,more preferably no larger than 8 cm.

The information storage medium 110 according to the present invention isresistant to environmental degradation and is preferably able towithstand temperatures between −273° C. (0° K) and 1200° C. withoutsuffering information loss. The information storage medium 100 may alsoresist electro-magnetic pulses, water damage, corrosion, acids and/orother chemicals. It is envisioned that the information storage medium100 as herein described could preserve information 120 for a time periodof at least 10 years, preferably at least 100 years, preferably at least1,000 years, more preferably at least 10,000 years, more preferably atleast 100,000 years. Under certain conditions of storage, includingstorage of the information storage medium 100 within an underground saltdome, the information storage medium may be able to preserve informationfor at least 1 million years.

FIG. 4 schematically depicts a cross section through an informationstorage medium suitable for long-term storage of information accordingto a further preferred embodiment of the present invention. Theinformation storage medium comprises a ceramic substrate 150 coated withfour layers 171 to 174 of different second materials being differentfrom the material of the ceramic substrate 150. Again, a sinteredinterface (not shown) may be present at least between the ceramicsubstrate 150 and the bottommost layer 171 of the four layers. Thesintered interface may comprise at least one element from both thesubstrate material and the material of the bottommost layer 171. Similarto the embodiment shown in FIG. 1 , the information storage medium ofthe embodiment shown in FIG. 4 comprises a plurality of recesses 10encoding information on the information storage medium, wherein theplurality of recesses 10 have different depths and wherein each depthcorresponds to a predefined bit of information. Again, 16 differentdepths are shown in FIG. 4 corresponding to a 4-bit code.

However, different from the embodiment shown in FIG. 1 , in case of theembodiment shown in FIG. 4 four different bits are encoded (by means ofdifferent depths) in each of the four layers 171 to 174. If the fourlayers 171 to 174 are made from different materials, the opticalresponse of each layer may be different. This allows for achieving highaccuracy during decoding because the depth information achieved may becorrelated with, for example, the optical response.

Of course, more or less than four layers of different second materialsmay be present depending on the number of bits to be encoded.

One particularly preferred example for the multi-layer coating shown inFIG. 4 is a two-layer coating with a metal layer 171 being coated on thesubstrate 150 and a metal oxide layer (of the same metal) 172 beingcoated on the metal layer 171. If such a two-layer coating isilluminated with incident white light as schematically shown in FIG. 5 ,a part of the incident light 1 is reflected (2) at the oxide layer,whereas another part of the incident light 1 is refracted (3) into theoxide layer and reflected (4) at the oxide/metal interface. The lightbeam having been reflected at the oxide layer and the light beam havingbeen reflected at the metal layer can be in phase, which leads to avisible colour, or out of phase, which does not yield said colour to bevisible. Accordingly, a certain colour (which depends on the indices ofrefraction of both the oxide layer and the metal layer and the thicknessof the oxide layer) is visible wherever the oxide layer is present, yetis invisible if the depth of a certain recess leads to destructiveinterference at this particular spot.

FIG. 6 depicts an exemplary graph showing the reflectance of laser lightdepending on the wavelength for a Ti/TiO₂ double layer with differentTiO₂ layer thicknesses (17 nm, 24 nm, 28 nm, 31 nm, 40 nm and 46 nm). Ascan be seen in FIG. 6 , the minimum reflectance depends strongly on thelayer thickness and is shifted from about 400 nm (thickness of 17 nm) toabout 700 nm (thickness of 46 nm) changing the color impression fromyellow to blue. Accordingly, a whole color spectrum may be encoded withmultiple recesses of different depths corresponding to the respectivereflectance minima.

Thus, it is in principle possible to create a polychrome microfilmutilizing a metal/metal oxide layer system and encoding differentcolours by means of different depths of recesses.

FIG. 7 schematically depicts a cross-section through an informationstorage medium suitable for long-term storage of information accordingto a further preferred embodiment of the present invention. Theinformation storage medium comprises a ceramic substrate 150 coated witha layer of the second material 170. Again, a sintered interface (notshown) may be present between the ceramic substrate 150 and the layer ofthe second material 170, wherein the sintered interface comprises atleast one element from both the substrate material and the secondmaterial. The surface of the layer of the second material 170 comprisesa plurality of nanostructures 20, wherein the plurality ofnanostructures 20 have different optical properties and wherein eachoptical property corresponds to a predefined bit of information. In thespecific example shown in FIG. 7 , the different optical properties ofthe plurality of nanostructures 20 correspond to different orientationsof so-called nano-ripples. In the depicted example, four differentorientations of such nano-ripples are shown corresponding to a 2-bitcode. Such nano-ripples having different orientations can bemanufactured as follows: A femtosecond laser can be used to create wavynanostructures named nano-ripples on ceramic (e.g. CrN) or metallic (Cr)surfaces. Several dozen to hundreds of linearly polarized femtosecondlaser pulses and energy flow far below the ablation threshold generatethe above mentioned nano ripples parallel to the direction of thepolarization.

Several examples will be described in the following.

As a first example, a ceramic substrate made of Rubalit 708s containingat least 96% Al₂O₃ having the dimensions of 20 cm×20 cm available atCeramTec GmbH (Germany) was used as the raw material.

A plate of said ceramic substrate having the size of 10 cm×10 cm and athickness of 1 mm was coated with a layer of CrN using physical vapordeposition. For this purpose, the ceramic plate was mounted on anelectrically conductive plate made from steel with a size of 10 cm×10cm. The ceramic plate together with the electrically conductive platewas brought into a physical vapor deposition machine available fromOerlikon Balzers AG (Lichtenstein).

Physical vapor deposition was then performed using the enhancedsputtering process BALI-NIT® CNI from Oerlikon Balzers AG at a processtemperature below 250° C.

After the deposition, a layer of CrN with a constant thickness of 5 μmwas present on one side of the ceramic substrate (opposite to the sidefacing the electrically conductive plate).

Subsequently, the coated ceramic substrate was tempered in a batchfurnace model “N 150/H” available from Nabertherm GmbH. For tempering,the temperature was ramped up from room temperature (20° C.) to 1,000°C. within 2 h. The temperature was then increased with a rate of 100 K/hfrom 1,000° C. to 1,200° C. and the maximum temperature of 1,200° C. wasmaintained for 5 min. Subsequently, the substrate was cooled down with arate of −200 K/h over 6 h.

After tempering, the stack of material comprised the ceramic substratemade of Rubalit 708s containing at least 96% Al₂O₃, a coating layer ofCrN having a thickness of about 5 μm and a further metal oxide layer ofCr₂O₃ having a thickness of about 1 μm. Similar metal oxide layers havebeen described in Z. B. Qi et al. (Thin Solid Films 544 (2013),515-520).

The metal oxide surface had a green darkish, almost black appearance.

The surface of said stack of material was inscribed in thin lines of10-20 μm width of different depths using the femtosecond laser “CARBIDE”available from the company Light Conversion. The laser parameters usedfor inscribing were 230 fs pulse width, 515 nm wavelength, 60 kHz and100 kHz repetition rate.

The laser created recessions reaching several depth levels between 4 and10 μm dependent on the number of pulses used. FIGS. 8 a and 8 b showmicrographs of the surface of said probe taken with the Keyence VHX-7000high-resolution 4K microscopes at different magnifications (the bar atthe right bottom of the two micrographs corresponding to 1,000.00 μm and100.00 μm, respectively) with the depth (and width) decreasing from leftto right.

FIG. 8 c shows a 3D visualization of a section through the micrograph ofFIG. 8 b . As may be taken from said figure, each recession has asubstantially constant width and depth along its length. FIG. 8 d showsthe cross sectional height profile through a section of the micrographof FIG. 8 a . Again, the depth clearly decreases from left to right. Asis clearly visible, the depth of each recession can be controlled by thenumber of pulses used for inscribing with each pulse creating a depth of500-1,000 nm.

Interestingly, the edges of the inscription show no sign of moltencoating material (CrN and Cr₂O₃) due to the cold ablation effect(Coulomb explosion) of ultra-short pulses.

As a second example, the same stack of material as described in thefirst example was produced.

The surface of said stack of material was inscribed in thin lines of1.92 μm width of different depths using a Spectra PhysicsFemtosecond-Laser Spirit-1040 HE30 (1040 nm, <400 fs, up to 120 μJ)using a focal length of 56 mm. Each laser pulse engraved a line recesshaving a depth of 1 μm. Each subsequent pulse at the same spot increasedthe depth by about 1 μm. Thus, five different line recesses of 1.92 μmwidth with depths of 1 μm, 2 μm, 3 μm, 4 μm, and 5 μm could be achieved.FIGS. 9 a and 9 b show micrographs of the surface of said probe takenwith the Keyence VHX-7000 high-resolution 4K microscopes at differentmagnifications (the bar at the left top of the two micrographscorresponding to 20 μm) with the depth increasing from left to right.

As a third example, a ceramic substrate made of Rubalit 708s containingat least 96% Al₂O₃ having the dimensions of 22 mm×7 mm available atCeramTec GmbH has been coated with 500 nm CrN in a Leybold Z400deposition system with the following process parameters:

-   3-inch Cr target (Plansee Composite Materials GmbH)-   base pressure below 5×10−6 mbar-   working gas pressure: 0.36 Pa with a N₂/Ar flow-rate-ratio of 16/16    sccm/sccm-   DC target power: about 200 W (current controlled with 0.5 A)-   no substrate heating-   no substrate bias (hence, floating potential).

The surface of said probe was inscribed in thin lines of 30 nm width ofdifferent depths using a FEI Quanta 200 3D DFIB (a focused ionbeam—FIB—workstation, equipped with a Ga ion source) with 6.667 nC/μm³at 0.1 nA and 30 kV (which corresponds to 2*1014 J/m³ or 0.2 mJ/μm³).The ion beam was focused to 11.5 nm spot size. The focused ion beamengraved in an initial passage a depth of 50 nm. Each subsequentinscription with a further ion beam passage increased the depth by about50 nm. Thus, ten different line recesses of 30 nm width with depths of50 nm, 100 nm, 150 nm, 200 nm, 250 nm, etc. could be achieved. FIG. 10shows an SEM image of the surface of said probe taken with a FEI Quanta250 FEG (a field emission gun scanning electron microscope—FEGSEM) withthe depth increasing from left to right. The spacing identified in theSEM image with two arrows corresponds to 30.0 nm.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and non-restrictive; theinvention is thus not limited to the disclosed embodiments. Variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art and practicing the claimed invention, from a study ofthe drawings, the disclosure, and the appended claims. In the claims,the word “comprising” does not exclude other elements or steps, and theindefinite article “a” or “an” does not exclude a plurality and may mean“at least one”.

1-60. (canceled)
 61. A method for storage of information, comprising:providing a ceramic substrate of a first material; coating the ceramicsubstrate with at least one layer each of a second material differentfrom the first material; and creating a plurality of recesses in the atleast one layer by using a laser in order to encode information in theat least one layer, wherein the plurality of recesses have differentdepths, and wherein each depth corresponds to a predefined bit ofinformation.
 62. The method of claim 61, wherein the ceramic substrateis coated with two or more layers, wherein the second materials of thetwo or more layers are different.
 63. The method of claim 61, whereinthe coated ceramic substrate is tempered before and/or after creatingthe plurality of recesses.
 64. The method of claim 61, wherein the firstmaterial comprises at least 90% by weight of one or a combination ofAl₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃, Zr₂O₃, or V₂O₃.
 65. Themethod of claim 61, wherein the first material comprises at least 90% byweight of one or a combination of a metal nitride; a metal carbide; ametal boride; or a metal silicide.
 66. The method of claim 61, whereineach second material comprises one or a combination of Cr, Co, Ni, Fe,Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, or V.
 67. The method ofclaim 61, wherein each second material comprises one or a combination ofa metal nitride; a metal carbide; a metal oxide; a metal boride; or ametal silicide.
 68. The method of claim 61, wherein the laser comprisesa femtosecond-laser, and wherein the plurality of recesses are createdas Coulomb explosions.
 69. The method of claim 61, wherein each at leastone layer has a thickness no greater than 1 μm.
 70. An informationstorage medium, comprising: a ceramic substrate of a first materialcoated with at least one layer each comprising a second materialdifferent from the first material; and a sintered interface between theceramic substrate and a bottommost of the at least one layers, whereinthe sintered interface comprises at least one element from both thefirst material and the second material of the bottommost layer, whereinthe at least one layer comprises a plurality of recesses encodinginformation, wherein the plurality of recesses have different depths,and wherein each depth corresponds to a predefined bit of information.71. The information storage medium of claim 70, wherein there are two ormore layers, and wherein the second materials of the two or more layersare different.
 72. The information storage medium of claim 71, whereinthe two or more layers each have a thickness smaller than 100 nm. 73.The information storage medium of claim 70, wherein a minimum depthdifference between the different depths of the plurality of recesses isat least 10 nm.
 74. The information storage medium of claim 70, whereina minimum depth difference between the different depths of the pluralityof recesses is at most 500 nm.
 75. The information storage medium ofclaim 70, wherein the first material comprises at least 90% by weight ofone or a combination of Al₂O₃, TiO₂, SiO₂, ZrO₂, ThO₂, MgO, Cr₂O₃,Zr₂O₃, or V₂O₃.
 76. The information storage medium of claim 70, whereinthe first material comprises at least 90% by weight of one or acombination of a metal nitride; a metal carbide; a metal boride or ametal silicide.
 77. The information storage medium of claim 70, whereineach second material comprises one or a combination of Cr, Co, Ni, Fe,Al, Ti, Si, W, Zr, Ta, Th, Nb, Mn, Mg, Hf, Mo, or V.
 78. The informationstorage medium of claim 70, wherein each second material comprises oneor a combination of a metal nitride; a metal carbide; a metal oxide; ametal boride; or a metal silicide.
 79. The information storage medium ofclaim 70, further comprising an oxide layer on top of a topmost of theat least one layers.
 80. The information storage medium of claim 70,wherein each at least one layer has a thickness no greater than 1 μm.